This invention relates generally to diagnostic imaging and, more particularly, to acquiring and reconstructing image data of structures subject to physiological motion using a computed tomography (CT) apparatus.
Helical cardiac acquisition has historically had the problem of high radiation dose to patients and lack of arrhythmia management. During a helical scan, the projection of x-ray toward a patient and table translation through the gantry opening are continuous. A step-and-shoot (SAS) acquisition technique has been used to overcome some of the problems associated with helical scanning, such as by significantly reducing the dose by, for example, more than a factor of two over a wide range of heart rates when scanning the cardiac region. In SAS mode, the patient table remains stationary while the x-ray tube and gantry rotate about the patient and the projections are collected over a minimum half-scan range. The table is indexed between acquisitions to move the patient with respect to the x-ray tube and gantry.
Currently, for cardiac scanning the SAS mode and other CT cardiac acquisition modes typically use the patient's electrocardiogram (ECG) signal to control data acquisition and reconstruction. Based on the R-R intervals, either a percentage delay or a fixed delay is used to determine the optimal acquisition and reconstruction windows. The ECG, however, represents the electrical signal of the heart and not the mechanical state of the heart. As a result, suboptimal gating often results as the heart rate (and thus motion of the heart) varies from beat to beat, sometimes to a large degree. Due to the variation, artifacts can result during reconstruction. Artifacts may be seen as discontinuities or abrupt changes in vessels and other anatomical structures that do not reflect accurate anatomical structure. Artifacts may also result from other physiological motion, such as respiration and involuntary motions of the patient when imaging structures within the patient other than the heart.
Therefore, a need exists for improved acquisition and gating techniques to improve CT imaging of structures subject to movement within a patient.